effect of particle concentration on filtration efficiency of ...

OPHELIA, 19(2): 163-174 (December 1980)

EFFECT OF PARTICLE CONCENTRATION ON FILTRATION EFFICIENCY OF THE BAY SCALLOP ARGOPECTEN IRRADIANS AND THE OYSTER CRASSOSTREA VIRGINICA ROBERT

E. P ALMER'~ &

LESLIE

G.

WILLIAMS

College of Marine Studies, University of Delaware, Lewes, DE 19958, USA

ABSTRACT Filtration efficiency of the bay scallop Argopecten irradians and the oyster Crassostrea virginica was examined in the laboratory using dilute algal suspensions, over a range of concentrations from 0.88 to 10.89 mg wet algal weight· 1- 1 . Efficiency of retention was measured in flowing seawater for each of 8 size classes of particles (1.73 to 9.97 /Lm diameter) by comparing number of particles in both inhalent and exhalent water. Algal suspensions were composed of isogravimetric amounts of Dunaliella tertiolecta, Isochrysis galbana, Platymonas suecica, and Thalassiosira pseudonana. In low algal concentrations, A. irradians and C. virginica showed no change in filtration efficiency for particles larger than 7 and 3 /Lm in diameter, respectively. As algal concentration increased, A. irradians became more efficient in retaining small (2 to 4 /Lm) particles, due to increased mucus (pseudofecal) production. Conversely, as algal concentration increased, C. virginica periodically became less efficient in retaining small particles, probably due to changes in effective ostial size. A. irradians conditioned to feeding on a large (10 /Lm) alga showed the same filtration efficiency as scallops conditioned on a small (4 /Lm) alga. These results indicate that both species show adjustments of filtration efficiency in concentrations of particulate matter representative of coastal and estuarine environments, and that these changes are utilized to control the amount of food collected for ingestion.

INTRODUCTION The amount of food collected by filter-feeding bivalves depends upon both the pumping rate of the organism and the efficiency with which particles of various sizes are strained from suspension by the gills. In recent years, morphological and physiological studies have described the structure (Moore 1971, Owen 1974, Owen & McCrae 1976) and the particle-retention efficiency (Haven & Morales-Alamo 1970, Vahl 1972a, b & 1973a, b, M0hlenberg & Riisgard 1978) of the gill of several species of bivalves. This literature indicates that considerable variation in efficiency occurs in the Bivalvia and that a relatively • Present address: 950 25th St., NW, Apt. 109 North, Washington D.C. 20037, U.S.A.

164

R.E. PALMER & L.G. WILLIAMS

reliable correlation exists between gill efficiency and gill structure. In those bivalves, such as Mytilus edulis, in which eulaterofrontal cirri are well-developed (Owen 1974), all particles larger than 4 /Lm are strained with equal efficiency (M0hlenberg & Riisgard 1978). On the other hand, in those bivalves, such as Chlamys varia, in which laterofrontal tracts are composed solely of unbranched cilia (Owen & McCrae 1976), the gill is maximally efficient only on particles larger than 6 to 7 /Lm. Capacity of bivalves to vary the retention efficiency of the gill in response to environmental factors could serve several functions. In filter-feeding copepods, for example, spacing of the setal filter is dependent upon feeding history, with the result that type of food collected tends to be that which is readily digested by enzymes induced from previous feeding (Donaghay & Small 1979). Further, changes in retention efficiency of bivalves could prevent clogging of the gill in dense suspensions and also serve to regulate the amount of material retained by the gill (Bayne et al. 1976). Whether changes in retention efficiency actually occur in bivalves under natural conditions is not known. Variations in efficiency have been observed or inferred in many studies (Loosanoff & Engle 1947, Chipman & Hopkins 1954, Smith 1957, Chipman 1959, Davids 1~64, Dral 1967, Haven & MoralesAlamo 1970, Vahl 1972a, Wilson & Seed 1974, Jr;ngensen 1975, Bayne et al. 1976, Epifanio & Ewart 1977). However, none of these studies has demonstrated a quantitative relationship between filtration efficiency and specific environmental factors. In laboratory studies on bivalves (M0hlenberg & Riisgard 1978) and tunicates (Fiala-Medioni 1978a, b) in which undisturbed organisms fed in dilute algal suspensions, no significant variations in filtration efficiency over time were observed. In the present study, filtration efficiency of the bay scallop Argopecten irradians concentricus (Say) and the oyster Crassostrea virginica (Gmelin) were examined in the laboratory using dilute algal suspensions. Two sets of experiments were designed to study the effect of concentration of suspended matter on retention efficiency in scallops and in oysters. A third set of experiments examined the effect of food preconditioning on filtration efficiency in scallops. Cordial thanks are due to the many people who helped make the study possible: Michael Castagna and John Kraeuter, who provided the scallops; Charles Epifanio, for generous use of laboratory equipment; Christopher Valenti, for assistance in algal culturing; Robert Moyer, who built the experimental chambers and Mary Christman and Pamela Palinski, for drafting the figures. College of Marine Studies Contribution no. 145.

FILTRATION EFFICIENCY OF ARGOPECTEN AND CRASSOSTREA

165

MATERIALS AND METHODS Experimental animals Argopecten irradians concentricus, reared in a hatchery and subsequently released into enclosures in a shallow bay near Virginia Institute of Marine Science, Wachapreague, Virginia, laboratory, were collected in July, 1979. Age of all scallops was 8 months at time of collection. Mean shell height was 42 mm, mean wet weight 17 g. Crassostrea virginica were purchased locally and suspended from floating rafts in Broadkill River estuary, Delaware, for 6 weeks before being brought into the laboratory. Mean shell height for oysters was 65 mm, mean wet weight 30 g. Three separate groups of bivalves were acclimated in the laboratory at the College of Marine Studies, Lewes, Delaware, for 1 to 3 weeks. One group of scallops and one group of oysters were fed once dally with a suspension of a small diatom, Thalassiosira pseudonana (Hustedt) Haisle & Heimdal. The other group of scallops was fed daily with a suspension of a large flagellate, Platymonas suecica Kylin. Median cell volumes for T. pseudonana and P. suecica were 25 ,um3 and 500 fLm 3 , respectively. Seawater (32 %0, 21°C) for acclimation and experimentation was collected from Indian River Inlet, Delaware, at high tide and passed through sand and charcoal filters before use. Particulate organic carbon content of the filtered seawater was low, 25 to 60 fLgC . I-I (J. Sharp, personal communication). Algae

Four species of algae were cultured in 40-liter carboys at 18 ± 1°C under continu~us cool white fluorescent illumination of about 800 fL W . cm -2. Thalassiosira pseudonana were cultured in Guillard & Ryther'S (1962) {-medium; Dunaliella tertiolecta Butcher, Isochrysis galbana Parke, and Platymonas suecica were cultured in {-medium without silicate. Median cell volumes for these 4 species were 25 fLm 3, 170fLm3, 40 fLm 3 , and 500 fLm 3 , respectively. Seawater for algal culture was collected from Indian River Inlet, filtered through cartridge filters to remove particles larger than 0.5 fLm (nominal pore size), passed through an activated charcoal filter, and pasteurized. Carboys were bubbled gently with an air-C0 2 (2 %) mixture. Experimental procedures

Twenty-seven filtration studies were performed on 6 groups of bivalves: 1) scallops acclimated on Thalassiosira pseudonana and exposed to low algal concentrations (0.88 mg wet algal wt . 1-1); 2) scallops acclimated on T.pseudo-

166


nana and exposed to medium algal concentrations (6.08 mg . 1-1); 3) scallops acclimated on T. pseudonana and exposed to high algal concentrations (10.89 mg .1- 1); 4) scallops acclimated on Platymonas suecica and exposed to medium algal concentrations (5.16 mg .1- 1); 5) oysters acclimated on T. pseudonana and exposed to low algal concentrations (1.45 mg' 1-1); and 6) oysters acclimated on T. pseudonana and exposed to high algal concentrations (6.54 mg . 1-1). For measurements of filtration efficiency, an individual oyster or scallop was placed in a plexiglass chamber (Fig. 1). The chamber measured 22 X 9 X 7 cm and, with the outflow tube positioned 4 cm above the base, held approximately 800 ml. From a 38-1iter header tank, algal suspension passed through a flow meter (Gilmont F1300) into the plexiglass chamber at the rate of approximately 7.21· hr-l. Four plexiglass baffles were glued into the chamber in positions designed to minimize recirculation of water pumped from the organism's suprabranchial chamber. At the initiation of each experiment, the density and median cell volume of each algal species were measured with a Coulter Counter Model ZB fitted with a 70 /Lm aperture. Equal wet weight (by cell volume) of each algal species was then added to seawater in the header tank to produce the desired concentration (milligrams per liter), and the tank was subsequently refilled every few hours with fresh algal suspension. Algal wet weight was calculated by assuming that Baffles

Drains

26 em FIG. 1. Shows experimental setup. Water and suspended algae flow from a header tank (not pictured) through a flow meter into the square end of the chamber, where water is forced over a baffle to insure mixing of the algae. As the water passes the oyster, it flows through a gate and beneath another baffle to eliminate remixing. Bottom drain can be plugged to activate upper drain.


167

algae were neutrally buoyant in seawater (density = 1.02 g . cm- 3 ) and by converting algal cell volume (cubic micrometers) into weight units (milligrams). Analysis of the particulate content of water flowing past the bivalve showed that actual concentration of suspended particulate material was generally within 10 % of the desired value. Use of 4 algal species, supplemented by small particles from the seawater system, ensured that a broad size range of particles from 1.5 to 12 /Lm in diameter flowed past the experimental organism. Each organism was allowed to acclimate to the experimental chamber and the algal suspension for 3 to 5 hr. During this time, 2 plastic siphon tubes were positioned 3 em upstream from the animal's inhalent current and in the path of the animal's exhalent current, for simultaneous collection of samples representative of water entering and leaving the bivalve, respectively. Twentyfive ml samples were collected at the rate of about 5 ml . min-to Concentration of particles from each of 8 size classes (1.73 to 9.97 /Lm in mean diameter) was determined with the Coulter Counter, and the fraction of each size class retained by the bivalve was computed as 1 - C.jC j , where C e was the concentration of particles in the exhalent current, and C j the concentration in the inhalent current. In 25 of 27 experiments, maximal filtration efficiency was reached on 6.9 /Lm or smaller particles; previous studies have also shown that retention efficiency of 13 species of bivalves is maximal on 3 to 7 /Lm particles (M0hlenberg & Riisgard 1978). Therefore, it was assumed that percentage efficiency was constant for particles larger than 7 /Lm, and retention efficiency for any size class was computed as the fraction of that size class retained as a percentage of the fraction retained from the 9.97 /Lm size class.

RESULTS

Behavioral observations Several behavioral differences were observed between Argopecten irradians feeding on low concentrations of algae (0.88 mg· 1- 1 ), in contrast to those feeding on either medium (6.08 mg· 1- 1 ) or high (10.89 mg· 1- 1 ) concentrations of algae. For scallops in low concentrations, the shell gape was extremely wide, so that the guard tentacles on the two inner (velar) folds of the mantle (Gutsell 1930) did not overlap. In addition, the free margin of the gill usually protruded 1 to 2 cm beyond the posterior-ventral margin of the shell. No pseudofeces were formed in low concentrations. For scallops feeding in medium or high concentrations, guard tentacles on the two velar folds interdigitated to form a screen, and the free margin of the gill extended to, but not beyond, the shell edge. The shell did not gape as widely as in low concentrations, and periodic adductions of the shell aided in expulsion of pseudofeces from the anterior-ventral margin of the body.

168


Crassostrea virginica feeding in low concentrations of algae (1.45 mg' I-I) formed only feces, while those in high concentrations (6.54 mg . I-I) formed both feces and pseudofeces, in an estimated ratio of 60 %: 40 %. Filtration efficiency Table 1 summarizes the filtration efficiency of each group of experimental animals on each of 8 size classes of suspended particles. The same results are presented graphically in Figures 2-6. Filtration efficiency curves of Argopecten irradians and Crassostrea virginica feeding on low concentrations (0.88 mg . I-I and 1.45 mg . I-I, respectively) of suspended matter were markedly different (Fig. 2). C. virginica filtered 3.4 /-Lm particles as efficiently as it did 10 /-Lm particles and removed over 50 % of 1. 7 [Lm particles from suspension. On the other hand, A. irradians was maximally efficient only on particles larger than 7/-Lm; scallops removed only 50 % of 4 /-Lm particles from suspension and only 1.5 % of 1. 7 /-Lm particles. There were significant effects of concentration of suspended particles on retention efficiency of Argopecten irradians (Fig. 3). Scallops feeding in medium (6.08 mg' I-I) concentrations were significantly (p < 0.05) more efficient in filtering particles from the 4 smallest size classes (1.73 -3.45 /-Lm) than were scallops feeding in low concentrations (0.88 mg . 1-1). There were no significant differences in filtration efficiency on any size class of particles between Argopecten irradians acclimated on Thalassiosira pseudonana and those acclimated on Platymonas suecica (Fig. 4). For every size class of particles, the mean filtration efficiency of Crassostrea virginica feeding in low concentrations (1.45 mg' I-I) was higher than for oysters feeding in high concentrations (6.54 mg . I-I) (Fig. 5). However, none of these differences were statistically significant (p > 0.05), due to the great amount of variation in filtration efficiency within each size class of particles for oysters feeding in high concentrations (Table 1). The cause of this variability is apparent in Fig. 6, which shows 3 filtration efficiency curves for the same oyster feeding at high concentrations, at 1800,1830, and 2000·hr. The curves at 1800 and 2000 were similar to those for C. virginica feeding at low concentrations of suspended matter (Fig. 4) - filtration efficiency was maximal for all particles larger than 3.4 [Lm. At 1830, however, the oyster was leaky to particles smaller than 7 [Lm; only about 55 % of particles between 4.5 and 6.9 /-Lm in diameter were removed from suspension, and only about 20 % of the 2 /-Lm particles were filtered. Of 5 replicate samples of oysters feeding on high concentrations of algae, 3 showed the 'high' efficiency characteristic of oysters feeding at low concentrations (Fig. 2), and 2 showed 'low' efficiency. None of the oysters feeding on low concentrations of particles showed 'low' filtration efficiency.

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TABLE L Filrration efficiency of 6 oc:perimenral groups of scallops (Argopecten ;mJd;llns) and oysters (Cranos/rea virg;n;ca) on 8 siu classes of suspen. d«l panicles.. Each efficiency value is expressed as mean percent efficiency ± standard deviation. Number in paremhescs indicates number of replicates.

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Z m

~

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Experimental group Mean panicle diameter (,..m)

concentration

1.73 2.18 2.74 3.46 4.35 5.48 6.91 9.97

1.5 ± 2.6(4 ) 14.3 ± 24.8 (4 ) 1704 ± 19.8{4} 37.2 ± 8.4 (4 ) 70.3 ± 4.6(4) 83 .6 ± 6.6(4 ) 103.0 ± 1.8 (4 ) 100.0(4)

Mean panicle conc. (mg . 1- 1 )

Scallop;

low

0.88 ± 0.13 (4 )

Scallop; high conccntr::ltion

Scallop; medium concentration

Scallop; P/atymomu· acclimated

concentration

± 6.0(5 ) 7504 ± 5.3 (5) 90.6 ± 404 (3 ) 99.0 ± 1.1 (3) 100.0(5 )

35.8 ± 19.8 (4) 51.7 ± 13.0(4 ) 64. 4 ± 5.7(4) 6004 ± 12.5 (4) 67.9 ± 19.9(4 ) 10 1.2 ± 7.8(4) 99.7 ± 3.2 (4) 100.0 (4 )

25.5 ± I U (5) 31.0 ± 20.7(5) 44.1 ± 22.8 (5) 55.7 ± 14.2 (5) 66.9 ± 18.6(5) 87.8 ± 14.0(4) 96.5 ± 3.7(5) 100.0(5)

57.0 ± 17.9(4) 78.2 ± 12.6 (4 ) 83.1 ± 7.8(4) 96.1 ± 4.0(4 ) 101.1 ± 1.1 (4) 9304 ± 1.3 (4 ) 93.1 ± 1.5 (4) 100.0(4)

10.89 ± \.87 (5 )

6.08 ± 0.53 (4)

5.16 ± 1.10 (5)

lAS ± 0.27 (4 )

45.1 44.7 48.7 56.4

± 6.0(4 ) ± 14.2 (4) ± 17.6 (5)

Oyster;

low

Oyster; high concentration

50.6 51.8 62.2 73.5 74.3 86.0

± 13.3 (5) ± 21.1 (5)

± 15.9 (5)

± 26.7(5) ± 31.9(5 ) ± 14.1 (4) n.O ± D.9(5 } 100.0 (5) 6.54 ± 2.75 (5)

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DISCUSSION The retention efficiencies observed for Argopecten irradians and Crassostrea virginica feeding in low concentrations of algae agree well with efficiencies measured for other pectinids and ostreids. In this study, A. irradians and C. virginica showed no change in filtration efficiency with particles larger than 7 and 3 /Lm, respectively. With one exception (Chlamys islandica, Vahl1973 b), all other pectinids have shown a decline in retention efficiency on particles smaller than 7 /Lm (Chlamys opercularis, Vahl 1972b; Pecten opercularis and P. septemradius, M0hlenberg & Riisgard 1978). Other studies on ostreids have also shown no change in filtration efficiency with p;lrticles larger than 3 to 4 /Lm (c. virginica, Haven & Morales-Alamo 1970; Ostrea edulis, M0hlenberg & Riisgard 1978). The difference in retention efficiency between scallops and oysters may be related to the structure and function of the gill in these two groups. In Ostrea edulis eulaterofrontal cirri are complex and closely (1.5 to

172


2.5 /Lm) spaced, and overlap to form a screen across the ostia (Owen & McCrae 1976). In Chlamys varia, on the other hand, ostia, 14 to 18/Lm in diameter, are guarded only by prolaterofrontal cilia, 12 to 15 f-tm in length. Collection of particles for ingestion in C. varia is not by ciliary straining, but by way of water currents which flow dorsally in the U-shaped principal filaments (Owen & McCrae 1976). Thus, a larger percentage of 3 to 7 f-tm particles are likely to pass unfiltered through the ostia of scallops than of oysters. The effective ostial size of scallops was the same for organisms preconditioned to feeding on a large (10 /Lm) alga as for those preconditioned on a small (4 /Lm) alga. Thus, increase in efficiency of the food-collecting filter with algal preconditioning, as was observed for copepods by Donaghay & Small (1979) and inferred for Crassostrea virginica by Epifanio & Ewart (1977), does not occur in Argopecten irradians. It remains to be shown that any bivalve can alter the efficiency of the gill in response to changes in the predominant size class of particles in suspension. The increased retention of 2 to 4 /Lm particles by scallops feeding in medium and high 3:1gal concentrations was probably caused by increased mucus (pseudofecal) production at these concentrations. Owen & McCrae (1976) proposed an alternative method of particle entrapment in scallops than by water currents flowing in the -principal filaments: 'When the concentration of particles presented to the gill is increased, ... increased amounts of mucus are secreted by the glands .associated with the ordinary filaments and mucus, plus entangled particles, are carried ventrally to the free margins of demibranchs.' Quantitative results reported in this study dovetail with Owen & McCrae's (1976) observations on pseodofecal production, and suggest that, in particle concentrations exceeding 2 to 5 mg . 1-\ a significant percentage of particles normally passing through the ostia of scallops are trapped by mucus secreted on the frontal surfaces of the gills. Chipman & Hopkins' (1954) observations of declining filtration rates of Argopecten irradians in unreplenished algal suspensions may be explained by assuming that retention efficiency declined as algal concentration dropped below the level eliciting pseudofecal production. When exposed to high concentrations of algae, Crassostrea virginica showed significant changes in retention efficiency over short « 1 hr) time intervals (Fig. · 6). However, whereas retention efficiency increased for scallops feeding in high algal concentrations, for oysters feeding in high concentrations retention efficiency periodically decreased from that typically shown at lower concentrations. Production of pseudofeces continued throughout the time that the variable efficiencies shown in Fig. 6 were recorded. Thus, it is likely that the decreases in retention efficiency periodically observed in oysters feeding in high algal concentrations were due either to changes in beat amplitude of the eulaterofrontal cirri (DraI1967, Bayne et al. 1976) or to changes in ostial size, controlled by the gill musculature (Bayne et al. 1976). Haven & Morales-Alamo


173

(1970) also observed periodic variations in filtration efficiency for populations of C. virginica feeding in seston concentration~ of approximately 10 mg . 1- 1 . Such changes in porosity of the oyster gill in high concentrations of suspended matter may help to explain how oysters can show extremely variable rates of pumping and filtration while keeping the valves open (Loosanoff & Engle 1947, Palmer 1980a). M0hlenberg & Riisgard (1978) found no significant changes in filtration efficiency over time in experiments with 13 species of bivalves. This is not surprising considering that the cell concentration used in those studies « 10000 . ml- 1 ) was below the level eliciting production of pseudofeces. Changes in retention efficiency in this study were not observed when algal concentration was below 1.5 mg· 1-\ but were observed when algal concentration exceeded 5 mg . 1-1 • It is probable that regulation of filtration efficiency by Argopecten irradians and Crassostrea virginica does occur in nature, since in the coastal and estuarine areas which these species inhabit, concentration of suspended material often exceeds 5 mg· 1- 1 • In Canary Creek, Delaware, average concentration of particulate organic carbon alone averaged 3 mg . 1- 1 during summer (Palmer & Carriker 1979). Haven & Morales-Alamo (1970) recorded an average concentration of suspended matter of 10 mg .1- 1 for York River, Virginia. Both Argopecten irradians and Crassostrea virginica showed changes in retention efficiency in concentrations of suspended matter exceeding 5 mg . 1-\ and the two species may utilize these variations in efficiency in different ways to control the amount of food collected for ingestion. In A. irradians feeding in high concentrations, increased mucus production results in increased retention of small (2 to 4 p,m) particles. However, since more material is rejected as pseudofeces at these higher concentrations, and since filtration rate decreases with increasing algal concentration (Palmer 1980a), amount of food material ingested is probably kept at a relatively constant level regardless of ambient concentration of particulate matter. Thus, amount of food ingested probably depends more on percentage rather than on the absolute amount of food material in suspension. In C. virginica feeding in high concentrations, effective ostial size, and thus retention efficiency, vary periodically. These mechanisms may serve to prevent clogging of the gill while maintaining the amount of ingested material at a level compatible with continuous intracellular digestion (Palmer 1980b).

REFERENCES BAYNE, B.L., R.]. THOMPSON & J. WIDDOWS, 1976. Physiology: 1. In B.L. Bayne (ed.): Marine mussels: their ecology and physiology. 121-206 pp. Cambridge University Press. CHIPMAN, W.A., 1959. The use of radioisotopes in studies of the foods and feeding activities of marine animals. - Publ. Staz. Zool. Napoli 31 (Suppl.): 154-175.

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CHIPMAN, W.A. & J.G. HOPKINS, 1954. Water filtration by the bay scallop Pecten irradians, as observed with the use of radioactive plankton. - BioI. Bull. 107: 80-91. DAVIDS, c., 1964. The influence of suspensions of microorganisms of different concentrations on the pumping and retention of food by the mussel (Mytilus edulis L.). - Neth. ]. Sea Res. 2: 233-249. DONAGHAY, P. L. & L.F. SMALL, 1979. Food selection capabilities of the estuarine copepod Acartia clausi. - Mar. BioI. 52: 137-146. DRAL, A.D. G., 1967. The movements of the latero-frontal cilia and the mechanism of particle retention in the mussel (Mytilus edulis L.). - Neth.]. Sea Res. 3: 391-422. EPIFANIO, C. E. & J. EWART, 1977. Maximum ration of four algal diets for the oyster Crassostrea virginica Gmelin. - Aquaculture 11: 13-29. FIALA-MEDIONI, A., 1978a. Filter-feeding ethology of benthic invertebrates (Ascidians). IV. Pumping rate, filtration rate, filtration efficiency. - Mar. BioI. 48: 243-249. FIALA-MEDIONI, A., 1978 b. Filter-feeding ethology of benthic invertebrates (Ascidians). V. Influence of temperature on pumping, filtration and digestion rates and rhythms in Phallusia mamillata. - Mar. BioI. 48: 251-259. GUILLARD, R. R. L. &]. H. RYTHER, 1962. Studies of marine planktonic diatoms. 1. Cyclotella nana Hustedt and Detonula confervacea (Cleve). - Can. J. Micro. 8: 229-239. GUTSELL, ].S., 1930. Natural history of the bay scallop. - Bull. u.S. Bur. Fish. 46: 569-632. HAVEN, D.S. & R. MORALES-ALAMO, 1970. Filtration of particles from suspension by the American oyster Crassostrea virginica. - BioI. Bull. 139: 248-264. J0RGENSEN, C.B., 1975. On gill function in the mussel Mytilus edulis L. - Ophelia 13: 187-232. LOOSANOFF, V.L. & J. B. ENGLE, 1947. Effect of different concentrations of microorganisms on the feeding of oysters (0. virginica). - u.s. Fish. Wildl. Servo Fish. Bull. 42: 31-57. M0HLENBERG, F. & H. U. RIISGARD, 1978. Efficiency of particle retention in 13 species of suspension feeding bivalves. - Ophelia 17: 239-246. MOORE, H.J., 1971. The structure of the latero-frontal cirri on the gills of certain lamellibranch molluscs and their role in suspension feeding. - Mar. BioI. 11: 23-27. OWEN, G., 1974. Studies on the gill of Mytilus edulis: the eulatero-frontal cirri. - Proc. R. Soc. Ser. B, 187: 83-91. OWEN, G. & J.M. MCCRAE, 1976. Further studies on latero-frontal tracts of bivalves. - Proc. R. Soc. Ser. B, 194: 527-544. PALMER, R.E., 1980a. Behavioral and rhythmic aspects of filtration in the bay scallop Argopecten irradians and the oyster Crassostrea virginica. - J. expo mar. BioI. Ecol. In press. PALMER, R.E., 1980b. Intracellular digestion and its relation to feeding history in the oyster Crassostrea virginica. - BioI. Bull. Woods Hole. 45: 273-295. PALMER, R.E., 1980b. Behavioral and rhythmic aspects of feeding and digestion in the bay scallop Argopecten irradians and the oyster Crassostrea virginica. - Ph. D. dissertation, College of Marine Studies, University of Delaware, Lewes, Del., 119 pp. SMITH, R.J., 1957. Filtering efficiency of hard clams in mixed suspensions of radioactive phytoplankton. - Proc. natn. Shellfish. Ass. 48: 115 -124. VAHL, 0., 1972a. Efficiency of particle retention in Mytilus edulis L. - Ophelia 10: 17-25. VAHL, 0., 1972 b. Particle retention and relation between water transport and oxygen uptake in Chlamys opercularis (L.) (Bivalvia). - Ophelia 10: 67-74. VAHL, 0., 1973 a. Porosity 'of the gill, oxygen consumption, and pumping rate in Cardium edule (L.) (Bivalvia). - Ophelia 10: 109-118. VAHL, 0., 1973 b. Efficiency of particle retention in Chlamys islandica (O.F. Muller). - Astarte 6: 21-25. WILSON, ].H. & R. SEED, 1974. Laboratory experiments on pumping and filtration in Mytilus edulis L. using suspensions of colloidal graphite. - Ir. Fish. Invest., Ser. B, 14: 1-20.